Eric Schmidt, the executive chairman of Google, has headed to North Korea with Bill Richardson, the former governor of New Mexico.

Mr Richardson, speaking at Beijing airport before boarding, said the trip was a "private humanitarian mission, not connected to the US government". A US citizen is currently held awaiting trial in the country.

"We're going to be in Pyongyang, probably for two and a half days. We may go outside the city. We will find out when we arrive," he said.

Kenneth Bae, an American of Korean descent, is being held in North Korea and his son contacted Mr Richardson to ask for his help, the former governor said last week.

North Korea has in the past agreed to hand over detainees to high-profile delegations led by the likes of former US president Bill Clinton, and some observers suggested it may have requested Schmidt's participation in this case.

But the US State Department has voiced concerns about the trip, saying it was ill-timed in the wake of Pyongyang's widely condemned rocket launch last month.

Mr Richardson has been to North Korea a number of times in the past two decades and has been involved in negotiating the release of US citizens held in the isolated country.

He told CNN on Friday that he expected to meet several senior officials in North Korea, though talks with leader Kim Jong-un were "very doubtful".

He added that he hoped the trip would be "positive" and dismissed US concerns, saying it had already been postponed once at Washington's request and the State Department should not be "nervous".

Both he and Mr Schmidt would be travelling as private citizens, representing neither the US government nor Google, he said.

Mr Bae, who was arrested in November, entered the country as a tourist, according to the North's official news agency which said he had admitted committing a crime against the state.

Nolan Barkhouse, spokesman for the US embassy in Beijing, said that Mr Richardson's trip was unrelated to the authorities in Washington.

North Korea last month angered the US and others by launching a long-range rocket. It said the purpose was to put a scientific satellite into orbit but Washington and other nations called it a disguised ballistic missile test.